Am I done yet? LOL

That's a good sign when your wife get engrossed in the music.  My wife does that as well.

I honestly think that I'm at a pretty good stopping point.  I like the way my system sounds and I think any improvements would be very tiny and very expensive. 

The amount of money one can dump into an audio system can become a bottomless pit!!!  The question is, how deep are your pockets Lol!!!  Happy listening.    

It sounds really good, but could use some more bass weight. 
During my speaker auditions, I had a pair of MoFi 888 speakers here and the bass was incredible....but that was all I liked about them. I could use some more of that. 

I do have a SVS SB-5000 R|Evolution sub, which is great for the very lowest bass, but I'm thinking about a second smaller sub or maybe an amp with better bass performance?

Nice system. Very clearly well thought out. Having your wife pleased is really a great endorsement. Women have much better hearing and are extremely sensitive to very low levels of high frequency hash and any distortion in the treble. If you are not careful you can put together a very high end system that will drive women up the wall... and not in a good way. So this is a great endorsement. 

So, I' recommend treading very carefully lest you loose that. I recommend doing a lot of listening to better preamps and amps. I would recommend Audio Research, Conrad Johnson, and VAC. These will elevate the level of details and realism without introducing a hard  or brittle high end. I would bring them home before buying and make sure your wife feels the same way. 

My partner finally loved the sound of my system when I moved to Audio Research... before that the same thing... that sounds very nice... and left. 

OP, if bass is what you're wanting, subwoofers are the best way to achieve this no doubt.  I wouldn't go with a smaller sub.  Typically it's nice to match subwoofers so that their response is the same (even though room nodes will destroy this, it's nice to have baseline)

I have two subs in both my systems and it's a massive improvement over having one.  It evens out the frequency response.

Have you done any EQing/DSP for this system?  Often times bass response issues are because of room nodes which can be corrected by dialing in the peaks in the response.  It really is not difficult to run pink noise through an analyzer program like Room EQ and determine an appropriate EQ curve for your particular response (especially in the region below 200 hz!!)

Do NOT go down an equipment rabbit hole if you just need better bass.  The problem is that the ideal positioning for mids and treble is almost never the ideal position for bass.  If you can crossover less sub frequencies to your Dynaudios, you will also possibly hear an improvement from those as well because the sub frequencies aren't being handled by your amp, or the speaker woofers, or the internal crossovers.  I've noticed that clarity improves on my system when I engage my subs even though technically the DSP adds some distortion, that distortion is more than made up for the fact that the tower cabinets aren't vibrating from sub bass and the components aren't having to handle the toughest material (sub 80 hz)

Best of luck!  My vote is, go grab another SB 5000, and dial in that bass response to your satisfaction by using REQ and a room mic like a umik-1